1. Field of the Invention
The present invention relates to a signal generation device and a signal generation method that induces characteristics of second-order cyclostationarity in a communication system that carries out signal identification based on second-order cyclostationarity.
2. Related Background Art
In recent communication systems, communication has come to be carried out by recognizing communication status by gathering information on signals received by a terminal that has received a signal, analyzing the recognized communication status, and using signal transmission parameters that enable the attaining of communication quality (prescribed quality of a communication rate or error rate and the like) that is convenient for or desired by the terminal based on the results of that analysis. Moreover, studies have been conducted on environment recognition communication systems that carry out such communication particularly in the field of wireless communication. In addition, in environment recognition communication systems, gathering of information based on signal demodulation is considered to be typical for recognizing communication status at a terminal.
However, in the case of carrying out communication within the same area in which a plurality of systems is present having different communication methods, since signals cannot be mutually demodulated, although it is possible to recognize the state of “a signal being present” when a signal has been received, there is the problem of being unable to gather any information beyond that.
In contrast, techniques have been attracting attention that focus on signal statistics and gather information on received signals by calculating statistics. Among these techniques, studies have been conducted in particular on second-order cyclostationarity, which has fewer arithmetic operations. Second-order cyclostationarity refers to statistics that generate different characteristics for signals having different parameters, and the use thereof makes it possible to easily identify a plurality of signals having different communication methods. Thus, in the case of carrying out communication within the same area where a plurality of systems having different communication methods is present, in addition to information in the form of “a signal is present”, information as to “which signal belonging to which system” can also be obtained when a signal has been received. However, even if this technique is used, there was still the problem of the amount of information obtained being extremely small and being inadequate for recognizing communication status.
Therefore, a technique has been studied that enables communication equipment to artificially induce a signal that generates characteristics of second-order cyclostationarity, and transmit a larger amount of information by means of artificially induced statistics. With regard thereto, reference may be made to, for example, U.S. Patent Publication No. 2008-0026704 (to be referred to as Reference 1) and P. D. Sutton, K. E. Nolan and L. E. Doyle, “Cyclostationary Signatures in Practical Cognitive Radio Applications”, IEEE Journal on Selected Areas in Communications (JSAC), Vol. 26, no. 1, pp. 13-24, 2008 (to be referred to as Reference 2).
In Reference 1, a signal generation method is disclosed that imparts an ID to a wireless signal based on statistics. Paragraphs 64 to 73 of the specification of Reference 1 disclose a first wireless signal generation method that induces characteristics of cyclostationarity in a multi-carrier system by transmitting the same symbol for a plurality of prescribed subcarriers. In addition, paragraph 70 discloses a second wireless signal generation method that uses discrete sine wave symbols as transmitted symbols for a plurality of prescribed subcarriers. Moreover, paragraph 61 discloses a third wireless signal generation method that transmits duplicates of transmission data signals at different frequencies on a frequency axis. In Reference 2, a wireless signal generation method is disclosed that induces characteristics of cyclostationarity by duplicating data transmitted by a portion of subcarriers when using an Orthogonal Frequency Division Multiplexing (OFDM) system, and generating transmission signals by controlling input to inverse Fourier transform so that the same data is transmitted by other subcarriers as well.
The first wireless signal generation method in Reference 1 and the wireless signal generation method of Reference 2 generate signals having characteristics of cyclostationarity corresponding to a prescribed number of remote subcarriers by unifying the transmission signal of a certain subcarrier with signals transmitted by the prescribed number of remote subcarriers from that subcarrier. However, according to these inventions, since the types of cyclostationarity characteristics that can be induced is limited by the total number of subcarriers, there was the problem of it being difficult to transmit numerous types of information using characteristics of cyclostationarity.
In addition, in the second wireless signal generation method of Reference 1, since the transmission of discrete sine wave symbols is an element of this method, data cannot be transmitted by subcarriers used to induce cyclostationarity, thereby resulting in the problem of a large overhead relative to all signals.
Moreover, in the third wireless signal generation method of Reference 1, although transmission signals are duplicated and frequency-shifted signals are transmitted simultaneous to those transmission signals, in the case the transmission signals are generated by a signal generation method that uses inverse Fourier transform such as OFDM signals, orthogonality between subcarriers cannot be secured due to the frequency shift, thereby resulting in the problem of the potential for deterioration of demodulation performance on the reception side.